Pneumatic percussive rockdrills are well known. Such machines typically include an impact motor containing a piston, reciprocable within a housing and configured so as in operation to deliver repeated impacts to an end of a drilling tool. Pneumatic rockdrills are also usually equipped with rotary means to rotate the drilling tool. This rotary means may be either a separate pneumatic rotary motor or a mechanical coupling from the impact motor, such as the well known rifle bar mechanism.
Pneumatic percussive rockdrills are usually also equipped with a small diameter rigid tube passing from the rear of the machine to just short of a striking face of the drilling tool. This tube passes through a hole in the centre of the piston and is more or less concentric with a hole down the centre of the drilling tool. At the rear of the machine this tube terminates in an external hose nipple. During drilling a relatively low pressure water hose is attached to the nipple, and water is injected down the rigid tube and through the hole in the drilling tool. This water exhausts from the drilling tool adjacent to the point of rock breaking during the drilling process, and serves to suppress airborne dust and to flush the broken rock fragments out of the hole being drilled. Water injection is an integral part of the drilling process, for both functional, and health and safety reasons, and therefore most underground pneumatic rock drilling sites are provided with both compressed air and a relatively low pressure water supply.
In order to lubricate these rockdrills, oil is added to the compressed air supply, typically by a venturi-type oiler. A small amount of the airborne oil entering the rockdrill is deposited on the internal surfaces, ensuring adequate lubrication. This is the well known technique of oil mist lubrication. Apart from the air passages to and from the impact and rotary motors, various secondary passages or leak paths are provided to duct air, and thus oil, to any other locations within the rockdrill which require lubrication. The oil has a secondary function of preventing corrosion of the various rockdrill components.
A large proportion of the oil entering a rockdrill of this type leaves the machine suspended in tiny droplets in the exhaust air. This is a serious health hazard to persons close to such a machine. Additional disadvantages of passing copious quantities of oil through a rockdrill are the cost of the oil and, in certain mining applications, contamination of the ore.
Various designs aimed at reducing the amount of oil passed through a pneumatic rockdrill are known. U.S. Pat. No. 3,983,788 discloses an impact motor that has two separate air circuits, one oil free and the other oiled. An enlarged central head of the impact piston is arranged to have a noticeable annular clearance within a central zone of the cylinder bore, while elongated ends of the Impact piston are guided in close fitting bushings. As a result of the annular clearance the piston can be oscillated by an oil free air supply, while the guide bushes and ancilliary components are lubricated by the second, oil laden air circuit. The vast majority of the compressed air consumed by a pneumatic rockdrill is used to reciprocate the impact piston, thus by powering this part of the machine with oil free air the amount of oil mist exhausted is significantly reduced. A disadvantage of this method is the complexity of the dual air circuits.
U.S. Pat. No. 4,333,538 discloses the use of an oil separator in the air circuit, upstream of the impact motor. A large proportion of the incoming oil is separated from the air entering the impact motor, ensuring that the minimum of oil required for lubrication passes through the impact motor. The remaining oil and some air is ducted directly to the ancilliary components of the rockdrill such as the chuck bushing and ratchet mechanism. Although not stated as an object of this invention, the more efficient distribution of the oil ought to result in an overall reduction in oil consumption and hence a reduction in the exhausted oil mist.
Also well known in the rock drilling industry are water-hydraulic percussive rockdrills. These machines use high pressure water as the working fluid instead of mineral oil as in traditional hydraulic machines. Some of the water exhausted by these machines is injected down the hole in the centre of the drilling tool to perform the dust suppression and hole flushing functions. Various design techniques and material selections have evolved to allow these rockdrills to operate successfully without any oil or grease lubrication. The only lubrication necessary is provided by the working fluid—water, and the use of suitable materials ensures that corrosion is not a significant problem. As a consequence, water-hydraulic rockdrills are completely free of the previously mentioned drawbacks of oil mist lubricated pneumatic rockdrills.
A disadvantage of water-hydraulic percussive rockdrills is that they require a different infrastructure to that of pneumatic rockdrills.
It is an object of the invention to provide a pneumatic rockdrill which seeks to overcome the abovementioned disadvantages, or which at least provides a useful alternative to existing pneumatic rockdrills.